AQA A-Level Chemistry Paper 3, 2017: Question 22
1 mark · Easy difficulty · Multiple Choice
Identify which point on a titration curve for a strong alkali added to a weak acid represents a solution that can act as a buffer.
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Question text
22 The diagram shows a pH curve produced by adding a strong alkali to a weak acid.
Which point on the curve represents a solution that can act as a buffer?
[1 mark]
A
B
C
D
Mark scheme
Show the mark scheme
22 B
How to answer it
Titration Curves: Identifying the Buffer Region
This question assesses your understanding of pH titration curves for a weak acid–strong base titration, specifically the chemical composition of the solution at different stages and the identification of the buffer region (half-neutralisation point).
pH Curve Analysis for Weak Acid + Strong Base
Identifying which labeled point on the titration curve can act as a buffer
✅ Correct Answer
B [1 mark]
Point B is located in the buffer region (near the half-neutralisation point). At this point, partial neutralisation of the weak acid (HA) has formed its conjugate base (A⁻), leaving significant quantities of both unreacted HA and produced A⁻ in solution.
💡 Key Knowledge
- Acidic Buffer: Formed of a mixture of a weak acid ( HA ) and its conjugate base ( A⁻ ).
- Reaction taking place:
HA + OH⁻ → A⁻ + H₂O - At Point B: Only some of the weak acid has been neutralised, so both HA and A⁻ are present in substantial amounts. The curve is relatively flat here because additions of small amounts of OH⁻ cause minimal pH change.
- Half-neutralisation: At exactly halfway to the equivalence point, [HA] = [A⁻] , so pH = pKₐ .
📐 Breakdown of Every Point on the Curve
- Point A (Start): No alkali has been added yet ( Volume = 0 ). The flask contains only the un-neutralised weak acid ( HA ). Because no conjugate base ( A⁻ ) has been formed (aside from slight dissociation), it cannot act as an effective buffer.
- Point B (Buffer Region): Weak acid is in excess. The solution contains both unreacted HA and the conjugate base A⁻ produced by the reaction. This satisfies the precise definition of an acidic buffer.
- Point C (Equivalence Point): Located mid-way up the vertical section. The acid is fully neutralised: moles of HA = moles of OH⁻ added . The solution contains only the conjugate base salt ( A⁻ ) and water; all HA has been consumed.
- Point D (Post-Equivalence): Excess strong base ( OH⁻ ) has been added. The pH is governed entirely by the concentration of unreacted strong base.
🧠 Exam Technique
- Look for the plateau: On a weak acid–strong base curve, look for the shallow horizontal region before the sharp equivalence jump. This plateau is always the buffer region.
- Buffer definition check: Quickly remind yourself: "Does this point have both the weak species and its conjugate partner?" Only a mixture can buffer against both added acid and base.
❌ Common Errors
- Choosing Point C: Confusing the equivalence point (where stoichiometry is equal) with the buffer region. At C, all the weak acid has reacted away, so it cannot neutralize added base.
- Choosing Point A: Assuming a weak acid by itself acts as a buffer. A buffer requires significant concentrations of both the weak acid and its salt/conjugate base.
Topics
Physical Chemistry · 3.1.12 Acids and Bases
Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.